The human heart is divided into four chambers, two upper chambers called atria and two lower chambers called ventricles. The heart's function is to pump blood through the body's circulatory system. A healthy heart at rest typically beats between 60 and 100 times per minute and will pump over 1,800 gallons of blood per day. Each normal heart beat is the result of electrical signals generated at a precise area in the right atrium, called the sino-atrial node, the heart's natural pacemaker. These electrical signals cause a physical contraction of the atria, which pump blood into the ventricles. The electrical impulses then continue to the ventricles, causing them to contract and distribute blood throughout the body.
Arrhythmias, abnormal rhythms of the heart muscle, arise from numerous causes, including tissue damage due to previous heart attacks, congenital defects and certain diseases. Arrhythmias can originate in either the atria where they are generally not life-threatening, or the ventricles, where they can significantly interfere with the pumping of oxygenated blood and can therefore be life-threatening. During an arrhythmia, the heart beats either too slowly or too rapidly. An abnormally slow heart rate, generally defined as a heart rate below 50 beats per minute, is known as bradycardia. This condition is usually treated by implanting a bradycardia pacemaker, a device that monitors the heart and delivers electrical impulses when necessary to increase the heart rate. A more serious arrhythmia occurs when the ventricles beat at an abnormally rapid rate, a condition known as ventricular tachycardia. In ventricular tachycardia, abnormal electrical signals occur in the ventricles. When the ventricles beat at an abnormally rapid rate, they do not have sufficient time to fill with blood prior to each contraction and therefore less blood is pumped out of the heart. As a result, less oxygen is carried to the tissues and organs of the body. This lack of oxygen can cause dizziness, unconsciousness, cardiac arrest and, ultimately, death.
Episodes of ventricular tachycardia occur unpredictably and tend to become more serious over time. Ventricular tachycardia can progress to the most serious type of cardiac arrhythmia, ventricular fibrillation. In ventricular fibrillation, the heart's normal electrical impulses become disorganized and erratic. Unlike ventricular tachycardia, during which the heart continues to contract in an organized fashion, in ventricular fibrillation the heart quivers and ceases to pump blood. As a result, the individual's blood pressure falls to nearly zero. If ventricular fibrillation is not terminated quickly, the individual will experience a sudden cardiac death (SCD) episode during which the individual will become unconscious as a result of the heart's failure to pump oxygenated blood to the body's tissues and organs, and without prompt medical intervention, typically will die.
A well-known device for treating patients with arrhythmias is an implantable cardioverter/defibrillator (ICD) which is an electronic device that is implanted in the patient and is designed to monitor the patient's heartbeat and deliver electric pulses or shocks to the heart to terminate arrhythmias. A typical ICD system consists of a device for pulse generation, defibrillation leads and pacing/sensing leads. The pulse generator contains the battery and electronic circuitry that monitors the patient's heartbeat and delivers therapy upon detection of a ventricular tachyarrhythmia. The pacing/sensing leads are insulated wires that connect the pulse generator to the heart and allow the device to sense the patient's heartbeat. These leads also carry electrical pulses for pacing. The defibrillation leads carry electrical shocks to terminate ventricular tachycardia and ventricular fibrillation. The defibrillator is surgically implanted beneath the skin in the patient's abdomen and the defibrillation leads are typically either epicardial patch electrodes connected to the exterior of the heart or endocardial leads inserted transvenously into the chambers of the heart. An endocardial lead system may also include a subcutaneous patch electrode. An ICD system of this type is described in U.S. Pat. No. 5,014,701 to Pless et al., which is assigned to the assignee of the present application and which is incorporated herein by reference.
A number of different ICD systems and methods have been developed for delivering electrical shocks to a patient's heart in response to detected arrhythmias. These methods deliver specific waveform shapes or pulse sequences to the heart in order to treat the detected arrhythmia. Different therapies may be used to treat different arrhythmias. In the case of conventional defibrillation pulses, high voltage shocks are delivered to the heart. Typically these shocks deliver an energy of from about 10 to 40 joules at a voltage of from about 500 to 1000 volts. The amount of energy delivered is determined by the patient's physician, typically by programming the voltage for the shock and the duration of either a monophasic or biphasic waveform. The programmed voltage is calculated by determining the actual defibrillation threshold of the individual patient with testing during device implant and the addition of a safety margin. The safety margin is needed for a number of reasons. First, the impedance of the leads may increase with time and this will reduce the actual energy delivered to the heart for a given voltage. Second, the threshold for defibrillation of a spontaneous fibrillation may be different from the induced fibrillation which is terminated during testing. Finally, it is typical to be conservative with safety margins when a patient's life is at stake. While this technique for programming and delivering high voltage shocks to the heart has proved effective as a lifesaving procedure, it has many drawbacks. First, defibrillation shocks are extremely painful to the patient. The lower the voltage which is effective and safe the better. Second, the high energy delivery requirements necessitates relatively large batteries and capacitors to enable delivery of the large shocks and to allow for device lifetimes of up to five years or longer. The presence of large batteries and capacitors results in a device package which may be uncomfortable for the patient, particularly for abdominal implants. A smaller device may permit pectoral implants which are more desirable for a number of know reasons. Thus, lower voltage shocks are less painful and disruptive to the patient and lower energy requirements allow for use of smaller batteries and capacitors and thus smaller implantable devices. One way of achieving this is to minimize the safety margin while still assuring successful defibrillation. Thus, it would be desirable if the necessary safety margin could be minimized.
An important feature of an ICD is the arrhythmia detection system. One of the earliest techniques for detection is described in U.S. Pat. No. Re27,757 to Mirowski in which a pressure transducer is positioned in the fight ventricle of a patient's heart. When the sensed pressure falls below a preset threshold, the device determines the presence of an arrhythmia and a therapy is delivered. More recent ICD systems rely primarily on an evaluation of the sequence of cardiac event timing intervals from a sensed electrogram (ECG). Various algorithms are applied to the detected timing intervals to determine the presence of an arrhythmia. Ventricular fibrillation is typically detected based strictly on heart rate (or interbeat interval) while tachycardia is detected based on rate along with other parameters such as sudden onset, stability, sensed physiological activity (exercise) and ECG waveform morphology. Certain rate boundaries are programmed into the ICD for each patient and these boundaries set up specific detection zones. These systems are not entirely satisfactory because there is still difficulty in making certain determinations, such as for example distinguishing between ventricular tachycardia and sinus tachycardia. It would therefore be desirable to have another detection system which could be used independently or in conjunction with prior art detection systems.
It is known that when an ICD is implanted in each individual patient, the defibrillation threshold should be determined and the defibrillation therapy to be delivered should be programmed with a safety margin. This is necessary because the energy required to defibrillate the heart may change due to changes in lead impedance over time and differences in the defibrillation threshold for spontaneous fibrillation as opposed to the induced fibrillation which is used during testing at the time of implant. The defibrillation threshold may also be different because some fibrillation events in a patient may be more severe than other events. This may be due, among other reasons, to the different possible initiating events for a fibrillation episode. By characterizing the seriousness or severity of a fibrillation event, the energy delivered for defibrillation may be modified to reduce the energy delivered for less severe episodes. A great deal of information for characterizing a fibrillation episode exists in the ECG signal, if it can be extracted and analyzed.
The most common type of mathematical transform for studying signals is the Fourier transform. It looks at all the data available from a particular signal, image, etc. and finds the probability of any individual frequency occurring in the signal. In this way, a signal is represented as the sum of its frequencies. A large amount of data from a signal may be compressed and certain information which may be hidden in the data may be viewed from a different perspective. The power of this representation diminishes when the signal that one is trying to represent changes its character unpredictably during the course of the signal. Essentially, local information is lost when the global representation of a Fourier transform is attempted. An improved method of performing this type of transform is known as the window Fourier transform. With a window Fourier transform, the time series is divided into small windows in time or in space depending on the nature of the data. The transform is performed to obtain the Fourier spectra of the data at various windows. The problem with this technique is that the uncertainty principle begins to set in. The smaller the window, so designed to better handle the localization of the data, the worse its frequency information becomes.
The uncertainty principle can be minimized using the Gabor transform. This transform makes use of the theorem that the minimum uncertainty is achieved with the Gaussian window. Thus, instead of performing the transform to break the signal down into its basic frequencies represented by a time series of sine and cosine functions, a Gaussian function is used. This improves the Fourier transform but still cannot give the detailed information of the time series.
A relatively new mathematical tool for performing this type of signal analysis is the wavelet transform. Wavelets represent a signal in a way that provides local frequency information for each position within the signal. Thus, the wavelet transform can be used to extract information of the time series which is not restricted to the sine or cosine functions. Essentially, the investigator has the freedom to choose any function which is appropriate for obtaining the relevant information of the time series. The advantage is that the signal can be observed at any time scale, i.e., the technique can zoom in on the signal, up to its finest resolution.
It is an object of the present invention to provide a method of characterizing fibrillation.
It is a further object of the invention to provide an ICD utilizing more efficient defibrillation energy delivery.